Surface preparation of substrates for bonding together has been carried out in very different ways in the prior art. For example, it is known to pretreat surfaces which are to be bonded in suitably composed pickling baths. Also, corona pretreatment of plastic surfaces and low-pressure plasma treatment are known to be very effective methods of treatment. Clear improvements in bond strength can be obtained with such methods, but their application is associated with high expenditures.
It is further known to treat substrates to be bonded together with solvents or mechanically with grinding, with emery and sand-blasting being preferred versions of the latter. However, improved climate resistance of the bonded composites is not obtained with such methods.
EP-A-O 326 097 provides a process for the preparation of a substrate surface for bonding with a plastic in which an adhesion promoting layer is deposited on the substrate surface as a sand-blasting composition which, relative to the weight of the total sand-blasting composition, consists of:
(A) 0.01 to 90 wt. % of an optionally silanized material with a particle size &lt;5 .mu.m and a hardness greater than that of the substrate surface and/or
(B) 20 to 100 wt. % of a silanized, silicon-containing material with a particle size of 2 to 200 .mu.m, and
(C) a sand-blasting agent with an average particle size &gt;5 .mu.m as the remainder.
The thus obtained adhesion-promoting layer is optionally silanized.
A similar process is described in EP-A-0 444 701. In order to obtain a good coating of the desired layer with the disclosed process, it is necessary to blast the sand-blasting agent at an angle of 90.degree..+-.10.degree.. The coating decreases rapidly as the blasting angle decreases.
Disadvantages are associated with sand-blasting processes like those described above. For example, high expenditures are required for apparatus, and the creation of dust causes problems, since the removal of dust often requires costly measures.